New Main Rotor Drive

      I knew when I first considered the KISS kit that I wanted to eliminate the Speedway Quick Change (QC) spur gears and housing. With my seat in the recommended position, the QC gear box would extend through the seat back into the cabin 2" or more. It would be too close to my head (a safety and ergonomics concern), spur gears are notoriously noisy, especially right next to your ear, and the housing interfered with the forward cyclic cable routing and also made it more difficult to install a decent firewall and sound dampening.

      I really wanted a commercially proven, off-the-shelf replacement for the QC rear end, but nothing was available. The Speedway drive uses a standard DANA 30 41:10 ring and pinion plus a set of 24:23 spur gears for an overall 3.93 reduction ratio. I found a commercially available 3.91 ratio (43:11) DANA 30 ring and pinion manufactured by Precision Gear, then designed the housing and the pinion modifications to replace the Speedway drive.

      I'm really pleased with the way the final design turned out. It fits in the same space, and still uses the Speedway end bells and the KISS aluminum spool, input flange/40 tooth pulley and sprag clutch.

      (Note: You can click on these pictures to view in higher resolution.)

      The new housing is only 9" outside diameter. It requires less space than the original Exec 162F chain drive and oil bath, and won't interfere with the forward cyclic control cable routing.
      Here is a comparison of the new drive with the Speedway QC drive. The new drive eliminates the spur gear housing and pair of spur gears, the long steel input shaft and 4 sets of bearings. Even though it is thicker and has a steel bearing support flange, it still came out 12.7 lbs lighter than the Speedway housing. Given the steel bearing support flange and the thicker aluminum in the pinion area, I believe the new housing is probably 3 or 4 times stronger (stiffer) than the Speedway housing. The additional stiffness should help maintain proper gear alignment during high loads and prolong gear life.
      Here are the new drive parts. Clockwise from the housing are the new pinion with head and tail bearings, the head and tail bearing shims, and the oil seal, which press-fits into the end of the bearing retainer flange next to it.
      I had the housing milled from a 4" thick 6061-T651 Al billet. The machined recess on the left side (one of four) is just for weight reduction. Two 1/8" NPT holes on this side of the housing are for the oil level sight tube. There is a third 1/8" NPT hole on the opposite side of the housing for the thermocouple.
      The machine shop did a great job with all critical dimensions and with roughing out the housing in their CNC mill. They couldn't easily program the complex contours around the front though, so I used a belt sander for final shaping and a small orbital sander for the final finish.
      The Bearing Flange was machined from 4340 CrMo plate, then heat treated and tempered to 40 Rc. The pinion tail bearing and oil seal press-fit into the back side of the flange. The pinion and head bearing press-fit into this front center recessed area and then into the drive housing.
      Here is a close-up of the bearing retainer flange and housing. The two parts are held together by six 7/16" NCxNF 150kpsi studs set in the housing with 1/2" Helicoils and epoxy. I calculated about 20,000 lbf pull out strength and 14,000 lbf yield strength per stud. (The nuts in this photo were temporary.) On final assembly, I oiled and torqued the self-locking flange stud nuts to 62 ft-lbs ea, or 11,100 lbf preload per stud.

      I calculated the absolute maximum axial thrust load from the pinion to be about 4,400 lbf; any single stud should be able to handle this with an additional 1.5x safety factor.

      I had the Precision Gear DANA 30 11-tooth pinion gear shortened, re-splined for the KISS input flange, tapped for a 7/16" retaining bolt, then carburized and heat treated to 60 Rc. I used an ARP 200-2802 7/16"-20 190kpsi bolt, torqued by ARP's specification to 16,000 lbf preload, to compress the KISS splined input flange against the tail bearing. I used a compression sleeve between the tail and head bearings to keep only the proper amount of preload on the tapered rollers.

      I calculated the maximum tapered roller bearing separation forces to be about 1,775 lbf; the ARP pinion bolt should be able to handle this with an additional 8x safety factor.

      This is a close-up of the original Speedway pinion gear as received (manufactured by US Gear.) It makes a strong statement for magnafluxing your gears.
      This is the new pinion I used, made by Precision Gear. I was relieved to see that it appears quite a bit more polished than the original pinion.
      This is the new drive housing during final assembly. The 16 (8 on each side) 3/8" studs for the end bells are threaded and epoxied in place.
      The most critical requirement of the whole drive system is to establish and maintain correct gear spacing and alignment. If the gears are not installed correctly or if the housing flexes during use, it puts a lot of strain at some single point on the gear teeth, which will cause premature gear failure.

      This picture shows the final contact pattern on the ring gear after the pinion and spool shims were properly established. The contact pattern ranges from slightly low to ideal at different intervals around the ring gear. I hope to get a few thousand hours of useful life out of this ring and pinion set, but I do intend to inspect it frequently.

Last Updated: January 1, 2006
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